1. Field of the Invention
The present invention relates generally to CDMA (Code Division Multiple Access) mobile communication systems, and in particular, to a method for performing high speed power control.
2. Description of the Related Art
Power control is essential to the smooth operation of a CDMA system. Because all users share the same RF band through the use of PN codes, each user looks like random noise to other users. The power of each user, therefore, must be carefully controlled so that no one user is unnecessarily interfering with others who are sharing the same band.
The importance of power control in a CDMA system is best illustrated by way of example. Referring to FIG. 12, there is illustrated a single cell that has two hypothetical users. Examining the reverse link, which is often the limiting link in CDMA, it is shown that user 2 is much closer to the base station than user 1. If there is no power control, both users would transmit a fixed amount of power pt. If it is assumed, for example, that the difference in distance is such that pt,2 is 10 times more than pt,1 then user 1 would be at a great disadvantage. It is apparent that user 2 has a much higher signal-to-noise ratio (SNR), and as such enjoys a great voice quality. Geographical factors can also influence disparities between users. Irrespective of whether the disparity results from distance or geography, this inequity is known as the classic near-far problem. Closed Loop Power control is commonly implemented to overcome the near-far problem and to maximize capacity. Closed Loop Power control is where the transmit power from each user is controlled such that the received power of each user at the base station is equal to one another. When this occurs, a greater number of users can be accommodated by the system.
In closed Loop power control when the received signal strength from a mobile terminal is excessively large, a control signal for reducing the transmit power of the terminal is transmitted from the base station to the terminal. Likewise, when the transmit power of a terminal is small, a control signal for increasing the transmit power of the terminal is transmitted from the base station to the terminal. In so doing, the received signal strength is maintained at an acceptable level in the base station. It is to be appreciated that the method has equal applicability with respect to the reverse link (i.e., signals received by the terminal from the base station).
FIG. 1 is a schematic block diagram of the single cell described in FIG. 1 comprising a single base station 20 and a single terminal 10. Closed Loop Power control, as performed in the prior art, will be described with reference to FIG. 1.
Referring to FIG. 1, a signal strength measurer 12 of a terminal 10 measures the signal strength of a forward link channel, and reports the measurement to a power control bit generator 13. Then, the power control bit generator 13 compares the received signal strength with a reference strength and generates a corresponding power control bit. If the received signal strength is larger than the reference strength value, the power control bit generator 13 generates a power control bit for reducing the transmit signal of the base station 20. If, however, the received signal strength is smaller than the reference strength value, the power control bit generator 13 generates a power control bit for increasing the transmit signal of the base station 20. The generated power control bit is inserted into a transmit signal of the terminal and transmitted to the base station 20. The insertion is performed in a power control bit inserter 14 of the terminal 10.
A power control bit detector 25 of the base station 20 receives the signal on a reverse link (or forward link) from the terminal 10, and detects the power control bit from the reverse link. A power control bit processor 26 of the base station 20 processes the detected power control bit to adjust the strength of the transmit signal from the base station 10 (or terminal 10). Here, the power control bit processor 26 commands a transmit power controller 27 to increase signal strength if the transmit signal strength is to be increased from an analysis of the power control bit, and to decrease the transmit signal strength if vice versa.
The strength of the power-controlled forward link signal is measured by the signal strength measurer 12 of the terminal 10. Thus, a closed-loop power control for the forward link is achieved.
A controller 11 of the terminal 10 performs other tasks including generation of transmit and receive data bits, which are not shown in FIG. 1, while sequentially controlling the closed-loop power control procedure.
Significant factors which impact closed-loop power control include: 1) the rate of power control adjustments; 2) the time required to implement power control through a closed loop; and 3) how to insert a power control bit. These factors require careful consideration as they have great influence on both signal maintenance and service quality.
FIG. 2 illustrates power control as implemented in a conventional IS-95 CDMA system. With a power control unit time selected to be 1.25 msec., power control will be performed 800 times per second. A number of operations are performed in the 1.25 msec interval including; 1) measuring the strength of a receive signal; 2) comparing the receive signal strength with a reference signal strength; and 3) generating and inserting a power control bit for insertion into the forward traffic channel, as shown in FIG. 2. The insertion position of the generated power control bit, shown in diagram as 213, is determined by a long-period PN (Pseudo-Random) code within the first two thirds of the 1.25 msec interval. Further, the power control bit is transmitted for a pre-determined time. Thus, power control bit transmit time can be equally distributed when transmitting a power control bit to a plurality of terminals thereby reducing interference, which can be cause by the power control bit, between a transmit signal and other signals.
The conventional IS-95 power control method, however, suffers from certain shortcomings in that the power control speed is limited to 800 Hz and the time delay associated with closed-loop power control ranges from 1.25 to 2.5 msec.
The first problem (i.e., the 800 HMhz speed limitation) with the conventional power control limits both the period and extent of power change which can be controlled. As is well known in the art, the power change period is inversely proportional to the speed of a mobile terminal. That is, as the terminal moves at higher speeds, the power change period becomes shorter and thus power change interval becomes more frequent. As a result, a power control performed at a rate of 800 Hz is not effective for a signal of a mobile terminal moving at high speeds.
In addition, considering the closed-loop power control is based not on a current power strength but on a previous measured power strength, a longer time delay in the closed-loop power control leads to power control being applied to the previous signal. In this case, power control is nullified if power change becomes large. That is, as time delay increases and power change gets larger, information contained within a power control bit which depends on measured signal strength is less accurate because it loses its relation with the current signal strength, thereby increasing a change in signal strength.
Each of the aforementioned shortcomings associated with power control as practiced in the prior art serves to increase the change of signal strength thereby decreasing the service quality. Accordingly, it would be highly advantageous to provide a method for maintaining a high reference signal strength to keep service quality at a predetermined level.
An object of the present invention is, therefore, to provide a closed-loop power control method in a CDMA mobile communication system, in which power change can be controlled for a rapidly moving terminal by performing a high-speed power control at various speeds.
Another object of the present invention is to provide a power control method in a CDMA communication system, in which the change in signal strength can be followed with a minimum time delay by minimizing the time delay in a closed-loop power control.
A further object of the present invention is to provide a power control method in a CDMA mobile communication method, in which signal interference caused by power control bits can be minimized by equally distributing power control bits on a temporal axis with introduction of bit-unit frame staggering.
These and other objects are achieved by a high speed power control bit insertion method for regulating power control between a base station and a mobile terminal in which power control bits are inserted in appointed positions in a transmission signal and as a result the power control bit insertion cycle is reduced.
The power control bit inserting method of the present invention is intended to maximize time used for demodulating inserted power control bits in a receiver and frequency diversities, so that bit errors [possibly] generated from power control bits are reduced and thus power control accuracy is increased. In accordance with the present method, for a particular bit error rate, the energy allocated to power control bits can be reduced.
Another additional advantage of the method of the present invention is the ability to perform closed-loop power control at various speeds. This is achieved by differentiating demodulation of power control bits in a receiver.
In one aspect of the invention, a method for inserting a power control bit in a signal transmitted between a base station and a terminal comprises the steps of: setting a unit time to a power control group period; allocating n-bit data to each signal path, where each signal path is comprised of a real and imaginary part; dividing the power control group period by the number of signal paths; inserting a power control bit in each divided segment, so that the insertion positions in the real and imaginary parts of each signal path are spaced from each other by n/2 bits; and the start position of the real signal path of each frequency is inserted at an appointed bit position of the corresponding frequency.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.